305/60R18 vs 265/60R20
New overall diameter
32.52″
was 32.41″
True speed at an indicated 60
60.2 mph
speedometer reads low
Revolutions per mile
620
was 622 · −2
Switching from 305/60R18 to 265/60R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 305/60R18 | 265/60R20 | Change |
|---|---|---|---|
| Overall diameter | 32.41″ | 32.52″ | +0.11″ (+0.3%) |
| Sidewall height | 7.20″ (183.0 mm) | 6.26″ (159.0 mm) | −24.0 mm |
| Tread width | 305 mm | 265 mm | −40 mm |
| Circumference | 101.82″ | 102.16″ | +0.35″ |
| Revs per mile | 622 | 620 | −2 |
| Speedometer error | baseline | +0.3% | reads low |
Diameter overlay
305/60R18 265/60R20
Speedometer check
With 265/60R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.1 mph actual
35 mph shown
35.1 mph actual
45 mph shown
45.2 mph actual
55 mph shown
55.2 mph actual
65 mph shown
65.2 mph actual
75 mph shown
75.3 mph actual
Common questions
- Will 265/60R20 fit instead of 305/60R18?
- Good fit. Within the 3% rule — overall diameter changes only 0.3%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read lower than your true speed. Overall diameter goes from 32.41″ to 32.52″, a change of +0.3%. As a rule of thumb, under 3% is safe, 3–5% needs a clearance check, and over 5% is not recommended.
- How far off will my speedometer read on 265/60R20?
- At an indicated 60 mph you would actually be travelling about 60.2 mph — a +0.3% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 305/60R18 and 265/60R20?
- 305/60R18 turns roughly 622 revolutions per mile; 265/60R20 turns roughly 620. That is a change of −2 revolutions per mile, which is what shifts odometer and speedometer readings.